Process and apparatus for coating panels with resinous compositions

ABSTRACT

In a process and apparatus for coating a panel with a resinous composition such as varnish or paint, a coating material which is normally fluid but which hardens under ultraviolet radiation is used. A layer of the composition is deposited on a bed permeable to ultraviolet radiation, and the layer is irradiated through the bed to cause part of its thickness to set. Then the panel is located on the layer and irradiation is continued until the coating has hardened fully.

nlted States Patent 1 3,7l3,%5 Grecchi 51 Jan. 36, i973 54] PROCESS AND APPARATUS FOR R25,l64 5/1962 Cline ..117/93.31 x COATING PANELS WITH RESINOUS 3222:: 24:22: artist, 2/222: COMPOSITIONS 3:250:642 5/1966 Parasacco et al ..l56/23I X 76 Inventor: Gi i Grecchi vi 'g I'HAy 3,257,252 6/1966 Keel U: ..l56/247 X r amc e a. cadorago, Italy 3 328 225 6/1967 U b t l 156/247 X [22] Filed: 1 July 18, 1969 t [211 A 842,89? 52271113Zfl'Z/E'Zlliiliiififil Att0rney-Sughrue, Rothwell, Mion, Zinn & Macpeak [30] Foreign Application Priority Data [57] ABSTRACT July 20, 1968 Italy ..52533-A/68 a In a process and apparatus for coatlng a panel with a resinous composition such as varnish or paint, a coat- [52] [1.8. CI. ..l56/23l, 156/249, 156/272, g material which is normally fluid but which hardws 117/9331,264/22,204/l59.ll I 51 l U 1 under ultraviolet radiation 18 used.-A layer of the com- 1 l ft. 0 0 p o is deposited on a bed pe e to ultraviolet [58] held of Search "156/272 247; radiation, and the layer is irradiated through the bed 117/933, 93.31; 264/22; 20 /l5 to cause part of its thickness to set. Then the panel is located on the la er and irradiation is continued until y [56] References Cited the coating has hardened fully.

UNITED STATES PATENTS 4 Claims, 2 Drawing Figures 2,32l,939 6/1943 Quinn ..l l7/93.3l

PROCESS AND APPARATUS FOR COATING PANELS WITII RESINOUS COMPOSITIONS This invention relates to the art of coating panels with resinous compositions, this art being of particular importance to manufacturers of panels for furniture, interior decoration, doors, and so on. The invention has particular application to panels of wood or similar materials, but is not confined to them.

In the past it has been common to apply a resinous composition in fluid form by spraying or painting or the like. In some cases the coating so formed has been caused to harden by baking the coated panel, or by applying the composition in the form of two layers which react together to harden. The compositions have commonly been of a type which, on hardening, form polyurethane or polyester resins.

Coating techniques that have commonly been used in the case of wooden panels involve presandpapering of the panel, application of one or two coats of a primer, and drying, cooling and sandpapering of each coat. Only then is the panel prepared for its final coat of resinous composition, which is often a paint or varnish. This process requires elaborate and expensive equipment in the form of sandpapering machines, applicators, drying and cooling tunnels, and so on. The process is time-consuming and the consumption of resin, because of the need for two or more layers, is high. With a polyester resin about 500 grams of resin per square meter of coated panel are generally needed to give satisfactory results.

An object of the invention is to provide a coating process which reduces the time and labor required, as also the quantity of resin per unit of surface area to be coated and which, in the case of panels of wood or similar materials with a porous surface, disposes with the need for presandpapering while yielding a finished panel with a desirably smooth or otherwise homogeneous surface.

Another object is to provide simple and relatively expensive apparatusfor coating panels with a resinuous composition.

Broadly, one aspect of the invention consists in a process for coating a panel with a resinous composition, using a fluid resinous composition which is normally inert but which is capable of hardening under the action of ultraviolet radiation; and including the steps of forming a coating layer of the composition on a bed that is permeable to ultraviolet radiation; irradiating the coating layer with ultraviolet radiation through the bed to initiate hardening of the coating layer, starting from the interface between the coating layer and the bed and proceeding until the coating layer has hardened in a part of its thickness; contacting the panel to be coated with the freeasurface of the coating layer; continuing to irradiate the coating layer through the bed until the entire coating layer has hardened; and removing the bed from thecoated panel.

Another aspect of the invention consists broadly of apparatus for coating a panel with a resinous composition, the apparatus consisting in combination of means for feeding over a path a flexible bed permeable to ultraviolet radiation; means for forming on the bed a coating layer of a fluid resinous composition that is capable of hardening under the action of ultraviolet radiation; means associated with a section of the path for irradiating the coating layer through the bed with ultraviolet radiation; means associated with a following section of the path for contacting the panel to be coated with the free surface of the coating layer; and means associated with a later following section of the path for continuing to irradiate the coating layer through the bed until the coating layer has substantially entirely hardened.

With panels of wood or other porous material having air inclusions, the panel is preferably superposed on the coating layer. The air in the pores can then migrate in the panel instead of tending to evolve as bubbles in the coating layer, a tendency which occurs in prior techniques when the coating is applied from above to the panel. In this fashion the step of exposing the coated panel to allow the escape of vapors, necessary in certain prior techniques, is obviated.

The bed is preferably an elongated flexible web or sheet, so that the process can be carried out continuously by feeding the web through suitable winding structure. The web may, for instance, be composed of cellulose tri-acetate, which has favorable physical and chemical properties and which in particular does not react with resinous compositions.

Resinous compositions which harden under the action of ultraviolet radiation, for instance by polymerization or cross-linking, are available on the market. One suitable composition of the polyester type, for example, is distributed by Farbenfabriken Bayer A.G. under the trade name Roskydal U.V. 10. When a polyester is used, the wavelength of the ultraviolet radiation should preferably be approximately between 2,750 and 2,850 A. Even conventional black light or Woods light lamps can however be successfully employed; their emission range generally lies between 3,000 and 3,800 A A preferred embodiment of the invention is described below with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic side view of a panelcoating apparatus according to the invention, and

FIG. 2 is a macrographic diagrammatic view in section of a layer of resinous composition supported on a bed.

In FIG. 1, 10 and 12 denote winding-out and take-up rolls for a cellulose tri-acetate bed in the form of a long web 14. Rollers 16, 18 are horizontally spaced along the web path, a section 14a of the web extending between them. A device 20 of any suitable type applies to the web travelling over the roller 16 a thin coating layer 22 (FIG. 2) of a fluid resinous composition that sets or hardens under ultraviolet irradiation. In practice the layer 22 will normally have a thickness of the order p of tenths ofa millimeter.

A cluster 24 of ultraviolet tubes 26 is arranged beneath the web section 14A, these tubes extending transversally of the web and being selectively ignited; that is, they are ignited individually or in groups. Thus, as the web travels over the cluster 24, each incremental section of the resin layer 22 receives through the web 14 ultraviolet pulses deriving from the lamps 26. These pulses create in the thickness of the layer 22 a solidification front such as denoted by 28 in FIG. 2, which moves from the interface of the layer 22 and web 14 towards the free surface 22A of the layer 22. By providing the apparatus with means obvious to those skilled in the art for adjusting the rate of feed of the web and by operating a suitable number of lamps 26 the level of penetration of the solidification front 28 can be controlled, so that on leaving the cluster 24 the layer 22 is hardened or solidified to an intermediate level only, denoted by 30 in FIG. 2, and the portion 228 above the line 30 is still fluid. Obviously, the line 30 has been described herein merely by way of illustration as there is in practice no sharp boundary between the solid thickness and the fluid thickness of the layer 22. The coated web 14 is led over a roller 31, then travels obliquely downwards to a roller 32. The web section 148 between the rollers 31, 32 has its coated face turned upwardly and is slidingly supported by means of a stationary planar glass plate 34. A cluster 36 of ultraviolet ray tubes 38 is arranged beneath the plate 34 in order to irradiate the resin layer 22 through the plate 34 and web 14.

The roller 31 has a pressure roller 40 cooperating therewith. A conveyor (not shown) feeds wooden panels P, P to be coated between the two rollers 31, 40. Thus, on traveling between the rollers 31, 40 each panel contacts the free surface 22A of the resin layer 22. Under the pressure prevailing between the rollers 31, 40, the fluid portion 228 of the layer 22 is thoroughly bonded or interpenetrated with the grain or fiber of the wood and enters the pores in the latter, the previously hardened portion 22C preventing the grain, fiber and pores of the panel from affecting the form and appearance of the bottom surface 22D of the layer 22, which takes its form from the surface of the web 14. The panels then travel obliquely downwards together with the web 14 on the plate 34, the hardening of the layer 22 being meanwhile continued by the lamps 38 until the coating layer has solidified fully. On travelling over the roller 32, the web 14 is loosened from the layer 22, which is now firmly attached to its panel. To facilitate this loosening it may be necessary or desirable to apply periodically to the web 14 a thin film of an anti-sticking agent, such as silicone oil, particularly when the surface of the web is not specular or glossy. A cutter 42 severs the portion of the layer 22 connecting each panel with that immediately following. The web 14 may have a specular or a matte finish, imparting a similar finish to the layer 22. Sliding thereof over the plate 34 is preferably promoted by coating the plate with silicone. The tension in the web section 14B, deriving from the take-up roll 12, is reduced by the downward inclination of the section of the apparatus associated with the web section 148. The angle of inclination is preferably adjustable.

In a preferred embodiment of the invention, the rate of feed of web 14 is of the order of l meter/minute. The prehardening on section 14A is effected within 30-40 seconds by igniting a number of tubes 26 covering 50-70 centimeters of the length of the web. Consequently it will be sufficient for the cluster 24 to cover a maximum of a meter of the length of the web, or slightly more, in order to perform a wide range of processes. The necessary time for hardening of the layer 22 to be completed over the section 14B of the web is of the order of few minutes and the ultraviolet tube cluster 36 should therefore extend over a corresponding length. Typically, 4 minutes are sufficient; however, in order to perform a wide range of processes,

a cluster 36, preferably of an extra length of about 8-1 0 meters is employed, and some of the tubes 38 are extinguished when full power is not needed. It will be obvious that a single coat only is necessary in the process of the invention to provide a finished panel. Minor irregularities in the surface of the panel as presented for coating, which would require presandpapering in prior processes, can be accommodated while still yielding a coating surface which is perfectly homogeneous, corresponding to the surface of the web. The time, labor and amount of resin required to coat the panel satisfactorily are thus reduced compared with prior processes.

It will be understood that the invention is not limited to the specific embodiment shown, and many modifications are possiible without departing from the scope of the claims.

EXAMPLE Veneer-covered panels of a pressed wood agglomerate are used, having a thickness of 2 cm. The exposed face of the veneer is brushed to eliminate dust. The mean roughness value of said face amounts to about 0.1 mm.

The transparent web 14 is driven at a linear speed of L7 m/min. The fluid resin delevered to the coating device 20 consists ofa mixture of:

a. unsaturated polyester obtained from maleic anhydride, l,4-butylenglycol, propylene glycol and anhydride of tetrahydrophthalic acid; the ratio of free hydroxyl groups to free carboxylic groups is 1:1.2;

b. styrene monomer;

c. benzoin-type initiator in an amount of 1 percent by weight referred to the sum (a) (b).

The Gardner-Holt viscosity of the fluid resin amounts to Z5.

The distance between the edge of the coating blade in the device 20 and the web 14 is adjusted to 0.22 mm. The lamps used in clusters 24 and 36 are Philips TL 40W/08. The effective lengths of travel of the web 14 over the clusters 24 and 36 (i.e., the lengths of travel over which the resin is exposed to U.V. radiations) are adjusted to 0.85 m and 7 m, respectively. The location of the solidification level 30, at the moment in which the panel is coupled with the web between rollers 31, 40, is estimated at about percent of the thickness of the resin layer; any way, the layer is still fluid through a depth of about 0.06 mm (zone 228 in FIG. 2) while, on the other hand, the layer is easily detachable from the web by rubbing the former with a finger (which fact denotes that the bottom part 22C of the layer is already hardened). The panels are coupled with the resin layer on the web in the manner already described hereinbefore; after stripping of the web at the roller 32 the resin coating adhering to the panel exhibits a smooth specular surface and is sufficiently hard to withstand usual handling conditions (e.g., stacking). The average thickness of the coating is 0.018 mm. The coating may be regarded as fully hardened after 24 hrs storage.

1 claim:

1. A process for coating a panel with a resinous composition, using a fluid resinous composition which is normally inert, but which is capable of hardening under the action of ultraviolet radiation; and including the steps of forming a coating layer of said resinous comlayer hardens; and removing the bed from the coated panel.

2. The process of claim 1 in which the panel is of porous material and in which the panel is superposed on the coating layer.

3. The process of claim 1 in which the bed is a flexible web, and in which the web is fed substantially continuously along a path to permit panels in sequence to be coated on adjacent sections of the web.

4. The process of claim 1 in which pressure is applied between the panel and the bed after the panel has been contacted on the coating layer, thereby facilitating bonding of the coating layer on the panel. 

1. A process for coating a panel with a resinous composition, using a fluid resinous composition which is normally inert, but which is capable of hardening under the action of ultraviolet radiation; and including the steps of forming a coating layer of said resinous composition on a bed that is permeable to ultraviolet radiation; irradiating the coating layer with ultraviolet radiation through the bed to initiate hardening of the coating layer, the hardening starting from the interface between the coating layer and the bed and being such that only a part of the coating layer is hardened; contacting the panel to be coated with the unhardened surface of the coating layer; continuing to irradiate the coating layer through the bed until the entire coating layer hardens; and removing the bed from the coated panel.
 2. The process of claim 1 in which the panel is of porous material and in which the panel is superposed on the coating layer.
 3. The process of claim 1 in which the bed is a flexible web, and in which the web is fed substantially continuously along a path to permit panels in sequence to be coated on adjacent sections of the web. 